1. Field of the Invention
This invention relates generally to apparatus and methods for controlling dot matrix output devices, such as cathode ray tubes and dot printing devices, e.g. laser printers and the like, to enhance edge representation when an image of an object is converted from graphical/mathematical format to display or print device format (a series of dots/pixels). More particularly, the present invention is directed to an improved implementation of weighted gradient work theory and associated logic used to provide image edge transition smoothing of both gray scale and text images in an efficient real time image enhancement system.
2. Brief Description of the Prior Art
In the field of image processing, research efforts have concentrated in the area of improving visual effects of object representation on output devices. Edges of original image objects, such as text, curves, boxes, and others, can be approximated by either linear or exponential equations. When these objects are digitized and converted into a matrix format which the output devices can accept, the integrity of the edges is usually compromised to fit the image objects into the device grids. The resulting staircase effects are quite visual to human perception in most of the commercially available products which have either low or medium resolutions.
The approximated appearance can be enhanced by locating the jagged edge segments and correcting the segment transition, or by increasing the device resolution to minimize the visual distortion to the degree that the naked eye can not differentiate the transition. However, increasing device resolution requires larger memory storage for the bitmap representation. The extra cost incurred by the memory is not desirable for general commercial products.
U.S. Pat. No. 4,321,610 issued to Moore et al. and entitled "Dot Matrix Printer with Half Space Dot Capability", discloses a technique which utilizes a half-dot character memory matrix and a shift register to enhance edges by coupling a half dot with an edge dot. This method at best enhances the resolution by a factor of two at the expense of extra memory storage and emboldening result.
U.S. Pat. No. 4,847,641 issued to Tung and entitled "Piece-wise Print Image Enhancement for Dot Matrix Printers" discloses a template matching method to correct error cells. Tung's invention utilizes the more recent pattern recognition and template matching process generally known in the image processing field; it is by far the most efficient way to enhance jagged line appearance. However, template matching and the compensation cell method limits its feasibility to use in monochrome devices only. While gray scale devices may share many similarities with monochrome devices, Tung's method would not be able to identify different gray patterns.
In a recent study of image segmentation and improvement, reported in the textbook entitled Digital Image Processing by Rafael C. Gonzalez and Paul Wintz (Addison-Wesley Publishing Company, 1977), weighted gradient mask matrices are applied to identify segments of an object by detecting changes in brightness. Due to the nature of complex matrix multiplication, this approach is recommended for remote usage for military or research purposes where computation power is sufficient and time is not a concern, but never in real time applications where segment recognition and modification need to be accomplished before the next pixel is displayed or printed.
As disclosed in U.S. Pat. No. 5,029,108 issued to James C.Y. Lung and entitled "Edge Enhancement Method and Apparatus for Dot Matrix Devices" it has been found that with selective entries in the gradient mask matrices, a simplified formula can be implemented to use the weighted gradient mask matrices to enhance the output of an electrophotographic printing machine or computer monitor display. This approach serves as an improvement over the prior art template matching methods which are not flexible in enhancing the gray scale applications. In order for the weighted gradient matrices to operate on gray scale objects, the entries of the matrices would range from 0 to 1 in any fractional increment manner, theoretically speaking. However, in real-time applications where limited computation power is a constraint, having fractional entries for gradient matrices requires complete matrix operations, such as multiplication, which extend beyond the usage of simple AND and INVERTER logic and therefore impose a restriction of the usage of the apparatus.
As outlined in the examples of the Lung patent, by confining the entries to two discrete values, 0 and 1, the disclosed method and apparatus can enhance edges along segment transitions with simplified logic to reduce the computation complexity. However, there continues to be a need for a similar apparatus and method which preserves the simple logic manipulation with modified weighted-gradient-matrix operations but further includes means for achieving gray scale edge enhancement.